Consider disabling PayPal buttons until required fields are completed

[robincornett]

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...on the EDD site itself if you click on the PayPal buttons then close the dialog a few issues happen:

1. You get a bunch of EDD errors about fields being incorrect (for example if you have not filled in any of the fields at the checkout).
2. You get an endless spinner/error that does not go away until you change gateways again.

Originally posted by @davidsherlock in https://github.com/awesomemotive/easy-digital-downloads/issues/9092#issuecomment-1050740229

Looking over the PayPal developer docs, I think we could set up a listener for our required fields and disable the PayPal button until they have been filled out, which would be a less disruptive user experience, I think: https://developer.paypal.com/docs/checkout/standard/customize/validate-user-input/

[mihaijoldis]

Maybe I should open a new issue but it seems similar to this one. At the moment in 3.X version if you click the PayPal button to pay, the modal windows pops up and then you close it you get a weird error message displayed on the page above the button: https://a.supportally.com/lHzTgx

This was reported here https://secure.helpscout.net/conversation/1954893974/2062354?folderId=5747299 and saw another similar one earlier today.

[robincornett]

@mihaijoldis is that happening if PayPal is the only available gateway? That's my guess--I think we need to update the JS we changed for #8883. If so this should be logged as a new issue.

uta: Just confirmed behavior if PayPal is the only gateway and the transaction is cancelled. Opening a new issue.